1. Field of the Invention
The present invention relates to an optical pickup device compatible with plural kinds of optical disks, and more particularly to a multi-wavelength laser device capable of emitting laser beams with a plurality of wavelengths.
2. Description of the Background Art
In recent years, it has been actively tried to develop record media with increased record density and also recording and reproducing devices therefor. Digital versatile disks (DVD), for example, are being widely used in a variety of fields as record media for images and are expected to be in increased demand in the future. Therefore, there is a growing demand for developing an optical pickup compatible with a conventional compact disk (CD), a recordable and reproducible CD (CD-R, CD-RW), and a DVD.
At present, a semiconductor laser diode (LD) with a 650 nm wavelength band is used as a light source for reproduction of DVDs. An LD with a 780 nm wavelength band is used for CDs and CD-Rs. An LD with a 410 nm wavelength band is to be used for the next generation DVDs. There are differences between recording and reproducing standards on the CD, CD-R/RW and DVD, and it is impossible for only one type of an LD to cover such different standards. Thus, it is desired to install a pickup or pickups capable of reading and writing with laser beams of different kinds of wavelengths in order to ensure the recording and reproducing compatibility between various devices.
In this case, three pickups corresponding to respective wavelengths may be installed in a recording and reproducing device. However, installing as many as three pickups in a recording and reproducing device inevitably increases the size of the recording and reproducing device and also causes increase in costs. Therefore, it is preferable that one pickup enables reading and writing for three kinds of media. To this end, it becomes necessary to use a multi-wavelength laser device capable of emitting three wavelengths for an optical pickup. Specifically, it is desired to provide a semiconductor laser device in which an infrared semiconductor LD with a wavelength of 780 nm, a red semiconductor LD with a wavelength of 650 nm and a blue semiconductor LD with a wavelength of 410 nm are arranged proximate to each other.
A schematic perspective view in FIG. 18 shows an exemplary multi-wavelength semiconductor laser device formed by integrating three kinds of semiconductor LDs with emission wavelengths different from each other (see, for example, Japanese Patent Laying-Open No. 2000-174398). The device of FIG. 18 is a so-called hybrid-type multi-wavelength laser device in which LDs as separately fabricated are integrated together. More specifically, along a flat side surface of a heat sink 131, semiconductor LDs 132, 133 and 134 respectively having infrared, red and blue emission wavelengths are mounted in this order such that emission directions of their laser beams are substantially parallel to each other. Wires 135, 136 and 137 are connected to these LDs 132, 133 and 134, respectively, so that each LD can operate independently. Such a hybrid-type multi-wavelength laser device is advantageous in that a defective LD can be replaced with a normal LD, thereby preventing reduction in yields of the multi-wavelength laser devices.
On the other hand, it has not been considered how to arrange three or more LDs having emission wavelengths different from each other when they are integrated together. According to thought of the inventors, however, in the case of integrating a plurality of LDs for use, relation between the arrangement of a plurality of LDs and the optical system of the optical pickup should be taken into consideration and it is important to arrange a plurality of LDs appropriately.
Ideally, it is desirable to prepare one optical system corresponding to one wavelength of the laser, similarly as in the case where three pickups are installed in one recording and reproducing device. This, however, does not bring about the advantage of integrating a plurality of LDs in one pickup. Similarly as in a pickup equipped with an LD of only one emission wavelength, therefore, a single optical system is to be preferably used even in a pickup including a plurality of LDs. In the laser device with a plurality of LDs integrated, on the other hand, it is physically impossible to coincidently position the emission points of those LDs at one point. Therefore, it is difficult for a single optical system to perform the functions of a plurality of optical systems that are optimum for respective LDs with respective wavelengths.